Estimated input cost for a cold prefix and its warm-cache delta.
Both figures are USD, non-negative, and finite; incremental_cost_usd is
the part of cold_cost_usd that a cache hit would have avoided, so it
never exceeds it.
Keyword-only for the same reason as PromptCachePolicy: these are adjacent
bare floats, and transposing them positionally yields copy that reads fine
("may cost up to ~$0.02 ... roughly ~$3.40 more than a warm cache hit")
while being arithmetically impossible.
